1. Field of the Invention
The present invention relates to a braking energy control apparatus and method for a vehicle equipped with an electric motor, wherein a vehicle braking force is secured in the form of regenerative braking energy based on a motor and other forms of braking energy, such as friction braking energy.
2. Description of the Related Art
As a braking energy control apparatus for a vehicle equipped with an electric motor, an electric vehicle driving force control apparatus is disclosed in Japanese Patent Application Laid-Open No. HEI 7-99704. FIG. 9 shows a block diagram of an electric vehicle driving force control apparatus as disclosed in the laid-open patent application.
As shown in FIG. 9, the electric vehicle driving force control apparatus 301 includes a brake pedal 302, a plurality of brake mechanisms 303, an accelerator pedal 305, a regenerative braking force range switch lever 306, a battery 307, and a control device 308. On the basis of the amount of depression of the accelerator pedal 305 and the range position of the regenerative braking force range switch lever 306, the electric vehicle driving force control apparatus 301 causes the power operation of coaxially connected first and second electric motors 309, 310 through high-efficiency excitation, or causes the regenerative braking operation of the first and second motors 309, 310. When the brake pedal 302 is depressed, the electric vehicle driving force control apparatus 301 operates the brake mechanisms 303 using hydraulic circuits in accordance with the amount of depression of the brake pedal 302, so as to brake wheels 312 that are being rotated together with axles 313.
When the accelerator pedal 305 is depressed, the electric vehicle driving force control apparatus 301 power-drives the electric vehicle by driving the first and second AC motors 309, 310 on the basis of the DC current from the battery in accordance with the amount of depression of the accelerator pedal 305. When the accelerator pedal 305 is released from the depression to assume an off-state, the electric vehicle driving force control apparatus 301 detects a charged condition of the battery 307. If the battery 307 is in a chargeable condition, the electric vehicle driving force control apparatus 301 causes the first motor 309 and the second motor 310 to operate for regeneration. The power regenerated by the regenerative operation of the motors 309, 310 is stored into the battery 307. If the battery 307 is in a fully charged condition or in a nearly fully charged condition, the electric vehicle driving force control apparatus 301 causes either one of the first and second motors 309, 310 to operate for regeneration. By using the regenerated electric power, the electric vehicle driving force control apparatus 301 power-operates the other motor, thereby achieving regenerative braking, without charging the battery 307.
If the accelerator pedal is off or only slightly depressed while the brake pedal is off during the running of the vehicle, an appropriate amount of braking energy is needed. The related-art electric vehicle driving force control apparatus 301 obtains a certain amount of braking energy while the brake pedal is off, merely through the control wherein the off-state of the accelerator pedal is detected and then only a regenerative braking energy is applied. Therefore, the aforementioned related-art control apparatus has a problem that the achievable braking energy is limited to the amount of regenerative braking energy that can be achieved by the electric motors.